Actually, they do congregate, prosper and become more toxic and pragmatic. They become far more effective at acquiring new members thanks to the echo chambers forming in both sides.

This is not new and has been studied for a long time; this is how gangs works. This is how cesspools such as /pol/ became what they are.

The only option to fight bigotry is open, honest discourse. A bigot's biggest enemy is reason and facts. Doing the opposite only creates division and makes the problem far worse.

Just because their out of sight and out of mind doesn't mean they can't affect the world around them. If anything you just gave them free reign since none of your eyes will be on them, and if their as dangerous as you think they are you just handed over one of the best assets for an attack. The element of surprise.